The present invention relates to clutches and, more particularly, to a bi-directional electromechanical overrunning clutch for providing four wheel drive capability.
The increased demand in recent years for off-road and all terrain vehicles has led to tremendous developments in those types of vehicles. Many of the developments have centered around making the vehicle more adaptable to changing road conditions, e.g., dirt roads, pavement and gravel. As the road terrain changes, it is desirable to vary the driving capabilities of the vehicle to more efficiently navigate the new terrain. Prior four-wheel drive and all terrain vehicles were cumbersome since they required the operator to manually engage and disengage the secondary drive shaft, e.g., by stopping the vehicle to physically lock/unlock the wheel hubs. Improvements in vehicle drive trains, such as the development of automated systems for engaging and disengaging a driven axle, eliminated many of the problems of the prior designs. These automated drive systems are sometimes referred to as xe2x80x9con-the-flyxe2x80x9d four wheel drive. These systems, however, require the vehicle to be in either 2-wheel or 4-wheel drive at all times.
Generally, all four-wheel drive vehicles include a differential for transferring torque from a drive shaft to the driven shafts that are attached to the wheels. Typically, the driven shafts (or half shafts) are independent of one another allowing differential action to occur when one wheel attempts to rotate at a different speed than the other, for example when the vehicle turns. The differential action also eliminates tire scrubbing, reduces transmission loads and reduces understeering during cornering (the tendency to go straight in a corner). There are four main types of conventional differentials: open, limited slip, locking, and center differentials. An open differential allows differential action between the half shafts but, when one wheel loses traction, all available torque is transferred to the wheel without traction resulting in the vehicle stopping.
A limited slip differential overcomes the problems with the open differential by transferring all torque to the wheel that is not slipping. Some of the more expensive limited slip differentials use sensors and hydraulic pressure to actuate the clutch packs locking the two half shafts together. The benefits of these hydraulic (or viscous) units are often overshadowed by their cost, since they require expensive fluids and complex pumping systems. The heat generated in these systems, especially when used for prolonged periods of time may also require the addition of an auxiliary fluid cooling source.
The third type of differential is a locking differential that uses clutches to lock the two half shafts together or incorporates a mechanical link connecting the two shafts. In these types of differentials, both wheels can transmit torque regardless of traction. The primary drawback to these types of differentials is that the two half shafts are no longer independent of each other. As such, the half shafts are either locked or unlocked to one another. This can result in problems during turning where the outside wheel tries to rotate faster than the inside wheel. Since the half shafts are locked together, one wheel must scrub. Another problem that occurs in locking differentials is twichiness when cornering due to the inability of the two shafts to turn at different speeds.
The final type of differential is a center differential. These types of differentials are used in the transfer case of a four wheel drive vehicle to develop a torque split between the front and rear drive shafts.
Many differentials on the market today use some form of an overrunning clutch to transmit torque when needed to a driven shaft. One successful use of an overrunning clutch in an all terrain vehicle is disclosed in U.S. Pat. No. 5,036,939. In that patent, the vehicle incorporates overrunning clutches directly into the wheel hubs, thus allowing each wheel to independently disengage when required.
A bi-directional overrunning clutch is disclosed for controlling torque transmission between a secondary drive shaft and secondary driven shafts. The present invention, when used in a vehicle, provides four wheel drive capability in the event of traction loss on any primary drive shaft.
The overrunning clutch includes a differential housing with a pinion input shaft extending outwardly from the housing. One end of the pinion input shaft is engaged with the secondary drive shaft. The other end of the input shaft is located within the differential housing and includes an input gear. The input gear preferably engages with a ring gear rotatably disposed within the housing such that rotation of the input gear produces concomitant rotation of the ring gear.
A clutch housing is attached to the ring gear and includes an inner cam surface. At least one and preferably two races are located adjacent to the cam surface. Each race is engaged with an output shaft. The output shaft, in turn, is engaged with a secondary driven half shaft.
A roll cage is located between the race and the cam surface. The roll cage has a plurality of slots which are preferably spaced equidistantly about its circumference. Each slot has a roll located therein. The roll cage is movable with respect to the clutch housing and the races.
A first armature plate is located adjacent to and engaged with the roll cage so that the first armature plate rotates in conjunction with the roll cage. A first  coil is mounted within the differential housing adjacent the first armature plate. The first  coil is adapted to produce an electromagnetic field when energized which hinders the rotation of the first armature plate, thus causing the roll cage to drag with respect to the clutch housing. The dragging of the roll cage with respect to the clutch housing causes the rolls to engage the clutch housing and the race when the wheels on the primary drive shaft lose traction. When traction loss occurs, the rolls become wedged between the clutch housing and the races so as to provide driving engagement therebetween.
A  In one embodiment, a second armature plate is located adjacent the roll cage. A second coil is mounted within the differential housing adjacent to the second armature plate. The second coil is adapted to produce an electromagnetic field when energized to hinder the rotation of the second armature plate. This causes the roll cage to advance with respect to the clutch housing causing the clutch housing to engage with the races. In this mode of operation, the secondary driven half shafts and output shaft drive the pinion input shaft and secondary drive shaft, thereby producing engine braking.
In another embodiment, a third armature plate is located adjacent to the roll cage and a third coil is mounted within the differential housing adjacent to the third armature plate. The third coil produces an electromagnetic field when energized which hinders the rotation of the third armature plate. This causes the roll cage to move opposite the direction of rotation of the clutch housing to assist in disengaging the rolls from between the clutch housing and the races.
The clutch housing preferably has a plurality of toggle levers pivotally attached thereto that engage with pins mounted on the roll cage. The engagement between the toggle levers and the pins permits the roll cage to be advanced and retarded with respect to the clutch housing. The second armature plate engages with the toggle lever to advance the cage and the third armature plate engages with the toggle lever to retard the cage.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures.